Game design-wise, co-op 3D is a different beast. 2D allows multiple players to share the same screen, therefore "share the experience". In a full 3D game, you have to do split screen. Which is possible and has been done -- but it's very hard to make those games foster the same feeling of cooperation. You pay more attention to your own side/corner of your screen, instead of the big picture. Everyone's free to wonder off and do their own thing -- and so, they do. The only way 3D co-op is done well if when players are FORCED to work together, which generally requires puzzles. That's why Portal 2 could work in this environment. But other genres? I haven't seen it figured out yet.